1. Field of the Invention
This invention relates to a process for the production of synthesis gas by the parallel use of a heat exchange reformer and an autothermal reformer.
2. Description of the Related Art
Parallel reforming may be used to produce hydrogen streams and synthesis gas for a number of processes such as ammonia synthesis, methanol synthesis as well as the Fischer-Tropsch process. In a parallel reforming process, a desulphurised hydrocarbon feedstock, e.g. natural gas, is mixed with steam and the mixture divided into two parts that are reformed in parallel. One part is fed to a heat exchange reformer (HER) where it is steam reformed, usually in externally-heated catalyst-filled tubes, and the other part is fed to an autothermal reformer (ATR) where it is usually combined with an oxygen-containing gas and autothermally reformed. The two reformed gas mixtures are combined and used to heat the catalyst filled tubes in the HER. The combined gas is then often subjected to one or more cooling stages, which may include stages of steam generation and may include pre-heating the mixed feed to the HER and ATR. Such a processes is described for example in U.S. Pat. No. 5,011,625.
One of the necessary features of the parallel reforming scheme is that the gas exit the HER tubeside is at a lower temperature than the gas exit the ATR. This is because to reach the same tubeside exit temperature as the ATR, the HER would require an infinite surface area. Therefore, to remain a practical size and cost, the HER must have a lower tubeside exit temperature to provide a realistic heat transfer rate through the tubes. The lower temperature of the tubeside gas means that it will have a higher methane slip, which is undesirable; therefore to avoid this, additional steam is typically fed to the feed gas flowing into the tubeside of the HER. In addition, the temperature difference between the reformed gas exiting the shellside of the HER and the temperature of the mixed feed to the tubeside is a measure of the efficiency of the reforming scheme. The higher the difference, the less efficient the scheme and the larger the flow of oxidant required to be fed to the ATR to make up for the higher amount of heat being lost from the reforming system. This may not only be a penalty in terms of oxidant flow, but too high an oxidant flow may be undesirable for the downstream processes by making the stoichiometry of the gas less than ideal. Furthermore, to ensure that the HER tubeside gas can be heated hot enough to have a low methane slip; the flow through the tubeside must be much smaller than the flow through the heating side. The need to feed a much higher flow through the shellside of the parallel HER to create a high tubeside exit temperature also lowers the efficiency of the parallel scheme.